Photoshop CC Brush Not marking on Layer?

I'm working in Photoshop CC with Windows 7. When I duplicate the layer of my photo and add an inverted mask the bush tool wont mark on my photo. The bush is set to normal and opacity 100% and there is still no change whatsoever. This is very frustrating. Please can someone explain how to fix this? I've tried closing the program and restating my computer but still no change.